1. Field of the Invention
The present invention relates generally to rechargeable electrochemical capacitors and, more particularly, to electrodes for electrochemical capacitors having low internal resistance and high charge/discharge rates. Specifically, the present invention relates to improved capacitor electrodes having intermediate conductive layers and improved metal oxide inks.
2. Description of the Prior Art
Electrochemical capacitors are devices which store electrical energy at the interface between an ionically conducting electrolyte phase and an electronically conducting electrode material. Electrochemical capacitors are a class of high rate energy storage devices which use such electrolytes and electrodes of various kinds in a system similar to that of conventional batteries. The electrochemical capacitors, like batteries, are essentially energy storage devices. However, unlike batteries, capacitors rely on charge accumulation at the electrolyte/electrode interface to store energy. Charge storage in electrochemical capacitors therefore is a surface phenomenon. Conversely, charge storage in batteries is a bulk phenomena occurring within the bulk of the electrode material.
Electrochemical capacitors can generally be divided into one of two subcategories. Double layer capacitors involved those in which the interfacial capacitance at the electrode/electrolyte interface can be modeled as two parallel sheets of charge. Pseudocapacitor devices, on the other hand, are those in which charge transfer between the electrolyte and the electrode occurs over a wide potential range and is the result of primary, secondary, and tertiary oxidation/reduction reactions between the electrode and the electrolyte. These types of electrochemical capacitors are currently being developed for high pulse power applications such as in cellular telephones.
Most of the known electrochemical capacitor active materials for both cathode and anode structures are based on metallic elements such as platinum, iridium, ruthenium, or cobalt. These materials are generally quite expensive and pose a significant hurdle to the widespread commercialization of this technology. Moreover, electrochemical capacitor devices have also suffered from problems associated with the manufacture and packaging of such devices. It is the nature of electrochemical capacitors to require relatively small packages which preferably develop high pulse power spikes and require high charge/discharge rates. Prior techniques of assembling such devices typically increased the thickness of the device as well as the complexity of the manufacturing process. Increased complexity resulted in manufacturing defects which caused yield losses. Moreover, as the capacitor package became thicker due to processing, the introduction of electrode equivalence series resistance (ESR), in other words internal resistance, reduced the efficiencies of the fabricated devices as well as decreased the charge/discharge rates.
One previous approach to this problem was to fabricate the capacitor by placing the relatively thick cell or series of cells which made up the capacitor under high physical pressure. While this increased compression approach to fabrication reduced the internal resistance in the device, it created a whole new set of fabrication problems. Therefore, there remains a need to provide electrochemical capacitor devices which feature low internal resistance, thin electrode profiles and high charge/discharge rates without the inherent problems associated with high pressure containers and compression fabrication techniques for thicker electrode structures. The present invention addresses this significant problem.